1. Field of the Invention
The present invention generally relates to image sensors and methods of manufacturing image sensors, and more particularly, to methods of manufacturing microlenses and microlens arrays used, for example, in image sensors, and a method of manufacturing image sensors including the same.
2. Description of the Related Art
Basically, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Image sensors are widely used in the semiconductor industry and are typically classified as either charge-coupled device (CCD) image sensors or complementary metal-oxide semiconductor (CMOS) image sensors. The CMOS image sensor has several desirable attributes such as, for example, its ability to be mass produced through a wafer process, its low power consumption, its compatibility with peripheral circuits, and its applicability to single-chip technology. Due to the above-mentioned desirable characteristics of CMOS image sensors, CMOS image sensors are widely used in digital cameras, smart phones, personal digital assistants (PDAs), notebooks, security cameras, barcode detectors, high-definition television (HDTV) cameras and toys.
The image sensor is mainly comprised of a light-detector (typically, including a photodiode) for detecting light and also a logic circuit for processing the detected light and converting it into an electrical signal. Moreover, to increase the image sensor sensitivity to light, the fill factor (i.e. a ratio of a light-detecting area to a pixel area) of the image sensor should be high. However, the fill factor of an image sensor is limited by certain factors such as chip size, number of the pixels and the difficulty in removing the logic circuit section completely.
Accordingly, a concentrating technology, which changes the path of light that is incident upon the outside of the photodiode and which also condenses the light into the photodiode, has been developed. With the above technology, microlenses are used to concentrate the light. In addition, each microlens is assigned to a respective pixel and is typically made of a plano-convex lens having a constant curvature.
FIG. 1 is a cross-sectional diagram illustrating a conventional piano-convex lens having a constant curvature.
Referring to FIG. 1, a curved surface of the piano-convex lens is a portion of a circle, which is centered on an origin P.
In the above conventional method, a microlens of an image sensor is formed so that the center of the microlens is positioned over the center of a photodiode. However, there are difficulties encountered with the above conventional image sensor, in that because light through module lens (not shown) is incident perpendicularly to a light-receiving plane at the center of the microlens array and is concentrated on the center of the microlens array, a photodiode far from the center receives a relatively smaller quantity of the incident light than a photodiode at the center of the microlens array. Consequently a phenomenon known as shading occurs, in which outskirts of an image captured by the image sensor look darker.
FIG. 2 is a cross-sectional diagram illustrating the above-mentioned shading phenomenon.
Referring to FIG. 2, each of microlenses 220L, 220C and 220R is placed over respective photo detectors 210L, 210C and 210R. As shown in FIG. 2, the light through the module lens (not shown) is incident upon the microlens 220C at the center of the microlens array from a direction perpendicular to the light-receiving plane, while incident on the microlenses 220L and 220R at the outskirts of the microlens array from a direction tilted from the right angle. Thus, with photo detectors 210L and 210R which are located far from the center of the microlens array, the incident light is not concentrated on the center of the photo detector, but rather on a margin of the photo detector, thereby resulting in a decrease in the light-receiving efficiency of the image sensor. In addition, because the incident light through the module lens is concentrated on the center of the microlens array, the photo detectors 210L and 210R far from the center of the microlens array receive a relatively small quantity of incident light.
Thus, a conventional method for diminishing the above-mentioned shading phenomenon has been developed. This conventional method utilizes a method of moving the position of the microlens far from the center of the microlens array to the center of the microlens array. By moving the position of the microlens far from the center of the microlens array to the center of the microlens array, the incident light may be concentrated on the center of the photo detector far from the center of the microlens array, thereby enhancing the light-receiving efficiency of the image sensor. This conventional method, however, is difficult to apply to certain types of image sensors such as those for example, with a shared-pixel configuration, because the position of the microlens in these image sensors is difficult to shift properly.
In Japanese Patent Laid-Open Publication Number 1999-340446, entitled “Solid Imaging Device and Method of Manufacturing the Same,” a solid imaging device is described, which attempts to enhance the light-receiving rate of the imaging device by causing the formation of an asymmetric curvature of a microlens far from the center of the microlens array, thereby concentrating the incident light on the center of a photo detector. Further, the method of manufacturing the solid imaging device of Japanese Patent Laid-Open Publication Number 1999-340446 described heating a microlens array, then rotating it and utilizing a centrifugal force so that the microlens of the microlens array might have a higher asymmetry of curvature in situations which the distance between the microlens of the microlens array and the center of the microlens array is greater. However, the method of manufacturing the solid imaging device of Japanese Patent Laid-Open Publication Number 1999-340446 is difficult to apply to a process for manufacturing a microlens array. In particular, with the process of manufacturing the microlens array described in Japanese Patent Laid-Open Publication Number 1999-340446, a wafer must be cut by a unit of a module lens, and a piece of the cut wafer must then be rotated, thereby resulting in increased manufacturing costs and time in producing the image sensor.
Therefore, an efficient method for manufacturing a microlens, which provides a microlens having an asymmetric curvature is needed.